Personally I've glued 12mm tubes to the various pieces after carefully balancing the gimbal. Drilling holes and screwing them in place will also work, but I find a combination of glue and screws more secure.

The same goes for the Phantom side of the top mount - it can either be drilled and screwed in place, or glued to the body. Either way works, but it is crucial that the frame is secured tightly to limit flexing and vibrations.